An Overview of Photocatalytic Membrane Degradation Development

Environmental pollution has become a worldwide issue. Rapid industrial and agricultural practices have increased organic contaminants in water supplies. Hence, many strategies have been developed to address this concern. In order to supply clean water for various applications, high-performance treatment technology is required to effectively remove organic and inorganic contaminants. Utilizing photocatalytic membrane reactors (PMRs) has shown promise as a viable alternative process in the water and wastewater industry due to its efficiency, low cost, simplicity, and low environmental impact. PMRs are commonly categorized into two main categories: those with the photocatalyst suspended in solution and those with the photocatalyst immobilized in/on a membrane. Herein, the working and fouling mechanisms in PMRs membranes are investigated; the interplay of fouling and photocatalytic activity and the development of fouling prevention strategies are elucidated; and the significance of photocatalysis in membrane fouling mechanisms such as pore plugging and cake layering is thoroughly explored.


Introduction
Industrial organic pollutants such as hormones, pesticides, pharmaceutically active substances, and personal care products have become a growing environmental concern due to their continuous release into the waterways and subsequent detrimental effects on human health, plants, soil, and aquatic systems [1][2][3][4]. Over a long period, antibiotics in their active form (low doses) and their main components can alter microbial communities in bodies of water [5,6]. Studying water samples of wastewater treatment plants in Beijing, China, revealed high concentrations of tetracycline, sulfonamides, and quinolones [7].
To remove contaminants and lessen their adverse effects on the environment and human health, a number of purification approaches are available, such as chemical coagulation [8], biodegradation [9], flotation [10,11], absorption [12], and adsorption [13]. Despite their benefits, conventional procedures have drawbacks, such as their inability to entirely remove contaminants. For instance, there are a number of drawbacks to the adsorption technique used for wastewater treatment, such as the challenging nature of separating the adsorbent from the solution and the high cost of the adsorbent [14,15]. Therefore, it is crucial to develop a novel, incredibly efficient method to eliminate contaminants from wastewater before they enter the environment [16]. Both polymeric and mineral membranes have shown excellent self-cleaning and anti fouling properties through photodegradation when exposed to visible light and UV irra diation [37,38]. A photocatalytic membrane may utilize the absorbed energy from the ir radiation source and breaks down pollutants that adhere to its surface; thus, such a mem brane is self-cleaning and anti-fouling [38].
The main focus of this research is how to use PMRs to clean water and waste water The photocatalyst, membrane, and light source are the main components of PMRs that are hereby discussed in depth. Membrane fouling is a serious operational concern in slurry type reactors; thus, its effects on permeate flux and system efficiency are described in length, along with solutions for mitigating the impacts of membrane fouling. Several o the most crucial operational variables that affect PMRs performance are covered. The standards for creating and developing PMRs are provided. Lastly, the most recent devel opments in the utilization of visible light are also discussed, along with efforts to circum vent some of the inherent drawbacks of PMRs, such as a moderate loss of photoactivity constrained processing speeds due to mass transfer issues, membrane leakage, and unsat isfactory system lifetimes due to photocatalyst leaching.

Photocatalytic Degradation Mechanism
As schematically shown in Figure 2a, degradation reactions are driven by the elec trons transferred from the valence to the conduction band. Typically, a photocatalyst's band-gap energy (E a ) should be equal to or less than the emitted photon energy [39][40][41] Electron transfer results in formation of an associated hole (h VB + ) in the valence band [42,43] Electron-hole pairs promote both oxidation and reduction of the adsorbed layer by gen eration of radicals [17,44]. Radicals are active oxidizing and reducing species that attack to and degrade contaminants in the aqueous solution [45,46]. A substrate reduction po tential below that of hole (h VB + ) results in the oxidation of substrates in the valence band whereas a substrate reduction potential higher than that of electron (e CB − ) results in the reduction of substrates in the conduction band [47,48]. Hydroxyl radicals (• OH) are the main species responsible for the photodegradation of pollutants. Another oxidant is reac

Photocatalytic Degradation Mechanism
As schematically shown in Figure 2a, degradation reactions are driven by the electrons transferred from the valence to the conduction band. Typically, a photocatalyst's band-gap energy (E a ) should be equal to or less than the emitted photon energy [39][40][41]. Electron transfer results in formation of an associated hole (h + VB ) in the valence band [42,43]. Electron-hole pairs promote both oxidation and reduction of the adsorbed layer by generation of radicals [17,44]. Radicals are active oxidizing and reducing species that attack to and degrade contaminants in the aqueous solution [45,46]. A substrate reduction potential below that of hole (h + VB ) results in the oxidation of substrates in the valence band, whereas a substrate reduction potential higher than that of electron (e − CB ) results in the reduction of substrates in the conduction band [47,48]. Hydroxyl radicals (•OH) are the main species responsible for the photodegradation of pollutants. Another oxidant is reactive oxygen species (ROS), such as superoxide oxygen radicals (O •− 2 ) [47]. Figure 2b shows photocatalytic degradation of cefixime, as an example of pharmaceutical pollutants, by Fe 2 O 3 @TiO 2 [13].

Mechanism of the Membrane Filtration Process
Pressure, concentration, or electric potential differences are the driving forces of membrane separation techniques. PMRs operate by pressure and concentration differences. Generally, surface water recycling for non-drinking purposes involves the utilization of ultrafiltration (UF) and microfiltration (MF) membranes. Nanofiltration membranes (NF) with molecular weight cut-offs (MWCO) between 150 and 350 Da are suitable for wastewater treatment and environmental cleanups due to their excellent inorganic ion removal capabilities [21].
The contaminated feed solution passes through the membrane under pressure. Membrane in a PMR retains the photocatalyst and pollutants and preferentially permeates water [49][50][51] as depicted in Figure 3a. In photocatalysis conditions at PMRs, polymeris membranes are highly susceptible to (1) abrasion by the photocatalyst and (2) degradation assisted by hydroxyl radicals. Thus, inorganic membranes are preferable due to their superior chemical, mechanical, and thermal resistance. One drawback of inorganic membranes is their cost. A thinner membrane enhances permeate flux due to its lower hydraulic resistance. However, if the membrane is too thin, it may be vulnerable to damage. Average pore size diameter, chemical, mechanical and thermal stability, cost, and lifetime are other important parameters that must be tailored for each membrane application. A list of membranes used for water decontamination is reported in Table 1.  [13]. Adapted with permission from Elsevier. Copyright 2023.

Mechanism of the Membrane Filtration Process
Pressure, concentration, or electric potential differences are the driving forces of membrane separation techniques. PMRs operate by pressure and concentration differences. Generally, surface water recycling for non-drinking purposes involves the utilization of ultrafiltration (UF) and microfiltration (MF) membranes. Nanofiltration membranes (NF) with molecular weight cut-offs (MWCO) between 150 and 350 Da are suitable for wastewater treatment and environmental cleanups due to their excellent inorganic ion removal capabilities [21].
The contaminated feed solution passes through the membrane under pressure. Membrane in a PMR retains the photocatalyst and pollutants and preferentially permeates water [49][50][51] as depicted in Figure 3a. In photocatalysis conditions at PMRs, polymeris mem-   [59] As depicted in Figure 3 there are four main solute transfer mechanisms in membranes. The main transport mechanism for a membrane process is determined by relative magnitude of (1) average pore size diameter (d), (2) size difference of transferable and nontransferable molecules, (3) mean free path of transferable molecule (λ), and (4) pore network structure. According to Figure 3b, presence of large pores in the membrane (d/λ > 20) results in convective flux. Convective flux through membrane accelerates the process; however, convection mechanism is only applicable in PMRs when there is a large size difference between contaminants/photocatalyst and water molecules. If such size difference does not exist, membranes with smaller pore size are required which transport water molecules by permeation. The permeation rate is affected by various factors, including pressure and concentration gradient, size and shape of permeate, and pore size, thickness and chemical structure of membrane. There are two main forces that drive the permeation: hydraulic pressure, which pushes solvent molecules through the membrane, and osmotic pressure, which opposes the flow of solvent due to the presence of dissolved species in the wastewater [60]. The Knudsen diffusion mechanism is dominant when d/λ < 0.2 [61]. Knudsen diffusion occurs only during gas transfer across nano porous membrane; thus, it is useful in removing gases and volatile organic compounds from wastewater [62,63]. Molecular sieves may also be used to purify water [64]. Solution-diffusion is the main transport in dense membranes. Solution-diffusion does not apply to water purification; however, it is the main mechanism for hydrogen transport in Pd-based membranes [65].
During the filtration process, pollutants adhere to the membrane surface and decrease membrane permeability. This phenomenon is calling membrane fouling which is one of the most important challenges of membrane processes [66,67].
Adsorption, accumulation, and precipitation are three mechanisms that may simultaneously occur to produce fouling [68,69]. To improve membrane performance, its surface is modified to maximize its affinity towards the permeating solvent and minimize its affinity towards the fouling agents [70]. Chemical modification, UV irradiation, applied electric field, aeration, and plasma treatment may be utilized to tune hydrophilicity of the  [71]. Filtration at critical flux has a modest flux drop and minimized irreversible fouling; however, it reduces output. Finally, to maintain the performance of the membrane system, cleaning and maintenance strategies such as backwashing and chemical cleaning are employed routinely during operation [72,73].
water molecules by permeation. The permeation rate is affected by various factors, inclu ing pressure and concentration gradient, size and shape of permeate, and pore size, thic ness and chemical structure of membrane. There are two main forces that drive the p meation: hydraulic pressure, which pushes solvent molecules through the membrane, and motic pressure, which opposes the flow of solvent due to the presence of dissolved species the wastewater [60]. The Knudsen diffusion mechanism is dominant when d/λ < 0.2 [6 Knudsen diffusion occurs only during gas transfer across nano porous membrane; thu it is useful in removing gases and volatile organic compounds from wastewater [62,6 Molecular sieves may also be used to purify water [64]. Solution-diffusion is the ma transport in dense membranes. Solution-diffusion does not apply to water purificatio however, it is the main mechanism for hydrogen transport in Pd-based membranes [65

Characterization of Membranes
Membrane production procedure may include phase inversion, interfacial polymerization, and coating. The casting solution and cooling bath utilized in the phase inversion process have a substantial effect on the tortuosity, pore size distribution, pore network. morphology, microstructure, and mechanical properties. Fouling, conditioning, chemical exposure, disintegration, cleaning, the aging process can result in reversible or irreversible alterations to the physical and chemical properties of membranes; thus, membrane characterization is an essential part of membrane manufacturing and maintenance [74]. Characterization techniques such as XRD, SEM, TEM, TGA, DSC, BET, Zeta potential analysis, and FTIR may be employed. SEM images in Figure 4a,b show cross-section PES membrane and irregular porous surface of PVDF membrane, respectively [75,76]. TEM image of Figure 4c shows components of a hierarchical layer of a TiO 2 nanowire membrane [77]. electric field, aeration, and plasma treatment may be utilized to tune hydrophilicity of the membrane surface [71]. Filtration at critical flux has a modest flux drop and minimized irreversible fouling; however, it reduces output. Finally, to maintain the performance of the membrane system, cleaning and maintenance strategies such as backwashing and chemical cleaning are employed routinely during operation [72,73].

Characterization of Membranes
Membrane production procedure may include phase inversion, interfacial polymerization, and coating. The casting solution and cooling bath utilized in the phase inversion process have a substantial effect on the tortuosity, pore size distribution, pore network. morphology, microstructure, and mechanical properties. Fouling, conditioning, chemical exposure, disintegration, cleaning, the aging process can result in reversible or irreversible alterations to the physical and chemical properties of membranes; thus, membrane characterization is an essential part of membrane manufacturing and maintenance [74]. Characterization techniques such as XRD, SEM, TEM, TGA, DSC, BET, Zeta potential analysis, and FTIR may be employed. SEM images in Figure 4a,b show cross-section PES membrane and irregular porous surface of PVDF membrane, respectively [75,76]. TEM image of Figure  4c shows components of a hierarchical layer of a TiO2 nanowire membrane [77].

Configurations of PMRs
A typical PMR includes a light source, membrane, and photocatalyst [78]. The design and configuration of the PMR dictates process efficiency and controllability. PMRs are classified into two types based on the photocatalyst loading. As schematically illustrated in Figures 5 and 6, suspended photocatalytic membrane reactors (SPMR) are those in which the photocatalyst is suspended, whereas immobilized photocatalytic membrane

Configurations of PMRs
A typical PMR includes a light source, membrane, and photocatalyst [78]. The design and configuration of the PMR dictates process efficiency and controllability. PMRs are classified into two types based on the photocatalyst loading. As schematically illustrated in Figures 5 and 6, suspended photocatalytic membrane reactors (SPMR) are those in which the photocatalyst is suspended, whereas immobilized photocatalytic membrane reactors (IPMR) are those in which the photocatalyst is fixed on a carrier material such as quartz, stainless steel, glass, limestone, or zeolite [79]. When a photocatalyst is immobilized on a support, the active surface available to solution particles is drastically decreased, resulting in a loss of photoactivity [80]. The active surface increases significantly when the photocatalyst is suspended; however, after detoxification, the photocatalyst particles must be separated from the treated water. Table 2 shows the numerous applications of PMRs in wastewater treatment processes.
Benefits of PMRs over traditional photoreactors include: (1) the ability to regulate the residence inside the reactor; (2) continuous operation; (3) the containment of the contaminants and photocatalyst within the reaction environment; (4) enhancing process efficiency and stability; and (5) reduced reactor volume and operating costs [36,42,81].
when the photocatalyst is suspended; however, after detoxification, the photocatalyst particles must be separated from the treated water. Table 2 shows the numerous applications of PMRs in wastewater treatment processes.
Benefits of PMRs over traditional photoreactors include: (1) the ability to regulate the residence inside the reactor; (2) continuous operation; (3) the containment of the contaminants and photocatalyst within the reaction environment; (4) enhancing process efficiency and stability; and (5) reduced reactor volume and operating costs [36,42,81].

Photocatalytic Degradation of Pollutants
Many organic and inorganic substances, especially toxic or refractory substances are resistant to biological degradation. After the discovery of the photocatalytic splitting of water in 1972 by Fujishima and Honda [93], scientists and researchers turned their attention to semiconductor photocatalysts that could destroy resistant contaminants that were difficult or impossible to remove by other methods [94]. Photocatalytic degradation has emerged as one of the most sustainable, energy efficient, cost-effective and non-hazardous and environmentally friendly processes for contaminants removal from water which uses light as the energy source [95]. The catalyst's photonic activation mode, which replaces thermal activation, is the primary distinction between photocatalysis and traditional catalysis [96]. Photocatalysts do not contain heavy metal and do not require strong oxidants/reducing agents for activation. Photocatalysis degradation products are harmless [47].

Photocatalytic Degradation of Pharmaceutical Compounds
pH is one of the most vital factors that affects photodegradation efficiency. The efficiency of TiO 2 and ZnO nanoparticles in removing acetaminophen from the water was studies by Ahed et al. [97]. The outcomes of the study demonstrated that ZnO was more effective at eliminating acetaminophen. Ahed et al. proved that pH is an important factor in photodegradation. Their synthesized ZnO photodegraded 97% of acetaminophen at pH = 9 in 1 h of exposure. They demonstrated that at neutral pHs, photocatalytic degradation rates are higher than at acidic pHs. Sabouni et al. [98] investigated the elimination of progesterone, ibuprofen, and naproxen using ZnO photocatalyst. They studied the initial concentration of pollutants and the photocatalyst loading. They found that ZnO photocatalyst is quite efficient in eliminating all three pollutants. They reported progesterone, ibuprofen, and naproxen as having a degradation efficiency of 92.3%, 94.5%, and 98.7%, respectively. The degradation of paracetamol using TiO 2 and Fe 2 O 3 photocatalysts was studied by Abdelwahab et al. SEM, TEM, XRD, FTIR, Raman spectroscopy, and VSM analyses were used to characterize the synoecized photocatalysts. Their results revealed that paracetamol degradation increased when TiO 2 loading in the TiO 2 /Fe 2 O 3 composite was increased [99]. The FTIR spectra of iron glycolate, Fe 2 O 3 , and 50% TiO 2 /Fe 2 O 3 reported by Abdelwahab et al. are displayed in Figure 7a. The peaks at 3432 cm −1 and 1618 cm −1 are attributed to the O-H stretching and bending vibrations of adsorbed water or EG, respectively. Peaks between 2850-2950 cm −1 are indicative of C-H vibrations. Between 1120 and 1470 cm −1 , the CH 2 bending vibration peaks could be seen. C-O stretching vibrations were responsible for two sharp peaks at about 1050 and 1085 cm −1 . Peaks of the Fe-O stretching vibration were seen at the 467-660 cm −1 . All of the peaks related to the glycolate moiety have disappeared in the Fe 2 O 3 FTIR spectrum, which was obtained after the iron glycolate was calcined at 350 • C [99].
When the iron glycolate sheets are stacked, a prominent low-angle diffraction peak at 11° is observed. The diffraction pattern of the Fe2O3 product created by calcining the iron glycolate at 350 °C shows that the main diffraction peaks match with the α-Fe2O3 rhombohedral structural pattern. However, two peaks at 2θ of 30 and 43° that are either indicative of Fe3O4 or γ-Fe2O3 show that the acquired sample could be made up of α-Fe2O3 and other iron oxide crystal forms. the relative magnitude hematite (α-Fe2O3) and maghemite (γ-Fe2O3) and/or magnetite (Fe3O4) are 70% and 30%, respectively. Furthermore, the new peaks at 2θ of 25.2, 37.4, 48, and 54° of the TiO2/Fe2O3 samples can be correlated to the (101), (004), (200), and (105) planes of anatase TiO2, demonstrating that the TiO2 crystallites were placed onto the magnetic core Fe2O3. The average crystallite size of TiO2, calculated by Scherrer's equation is 12 nm for the anatase (101) peak [99].   b. a.  (200), and (105) planes of anatase TiO 2 , demonstrating that the TiO 2 crystallites were placed onto the magnetic core Fe 2 O 3 . The average crystallite size of TiO 2 , calculated by Scherrer's equation is 12 nm for the anatase (101) peak [99]. Table 3 shows the photocatalytic degradation of different pharmaceuticals. Clearly, heterogeneous photocatalysts with a variety of nanostructures can effectively degrade pharmaceuticals in aqueous solutions.

Photocatalytic Degradation of Dye Compounds
Dyes are routinely used in the textile, food, beverage, printing, and pharmaceutical industries. These compounds may be harmful and cancerous will barricade sunlight from reaching water bodies, impacting natural aquatic processes such as photosynthesis and other biodegradation operations [112,113]. Long-lasting and non-degradable colored pollutants must be removed before entering the environment since their entry leads to the aquatic ecosystem becoming toxic and dangerous to humans [114]. The photocatalytic process has been suggested as one of the more successful methods. In photocatalysis, the degradation is begun with OH radicals breaking the azo bond (-N=N-), which is one of the weakest chemical bonds in the dye molecules' chemical structure [115]. The process's intermediates will then undergo a radical chain reaction with the oxygen molecules, finally breaking down to produce water and carbon dioxide [115]. In recent years, numerous studies on the removal of contaminants from actual wastewater have been conducted. Table 4 illustrates the photocatalytic degradation of a variety of dyes.

Photocatalytic Degradation of Hydrocarbons
Industrial use of hydrocarbons inevitably contaminates natural waters through improper disposal or leaching from landfills, spills, or leaks in underground pipes. Hydrocarbon contaminants endanger human health if they enter drinking water. The presence of these contaminants in water hinders light penetration into the water and affects the diffusion/solubility of gases required for aquatic plant respiration which ultimately leads to plant death; therefore, it may affect the food supply chain.
Schnabel et al. [127] used semiconductor titanium dioxide to remove hydrocarbons. This study demonstrated that a variety of photocatalyst designs, when exposed to ultraviolet (UV) light, can remove the non-polar material in diesel fuel. They reported that the floating foam glass catalyst with TiO 2 coating reduces the concentration from an initial concentration of 668 mg/L to 329 mg/L in 16 h. The contaminant concentration is reduced by 401 mg/L and 55 mg/L when glass fiber and steel grit was used, respectively [127].
Nirmala Rani et al. [128] studied the elimination of three polycyclic aromatic hydrocarbons (PAHs) in mixed or separate states using titanium oxide photocatalysts and MS membranes under UV irradiation in a PMR. They reported the degradation efficiency of 100, 94.1, and 97% in an aqueous mixture containing 1000 µg /L phenanthrene (PHE), 5000 µg/L naphthalene (NAP), and 1000 g/L acenaphthene (ANA), respectively, after 180 min UV irradiation at photocatalyst loading of 0.5 g/L. When the compounds were used as a sole compound, the elimination percentages of PHE, NAP, and ANA were 99.3, 92.8, and 95.3, respectively, under similar operating conditions [128,129]. The photocatalytic degradation of several hydrocarbons is reported in Table 5.

Photocatalytic Degradation of other Pollutants
Photocatalyst have been used to degrade pollutants listed in Table 6. Pitchaimani Veera Kumar et al. [139] employed zinc oxide nano stars (ZnONSt) coupled with Ag and Pd to photocatalytically degrade herbicides and pesticides. Ag at ZnONSt and Pd at ZnONSt photocatalysts accelerated the degradation of existing pollutants as they facilitate the interfacial charge transfer process [139]. In order to remove the herbicide ametrine, Rodrigo Pereira Cavalcante et al. [140] utilized a titanium dioxide photocatalyst. They were able to completely remove the ametrine after 60 min irradiation of simulated sunlight (using a 1000 W Xenon lamp), and then they used 0.4 g/L of photocatalyst to detoxify the solution. Samsudin et al. [141] utilized BiVO 4 /g-C 3 N 4 integrated with Pt to purify poultry during sun light exposure. The as-synthesized photocatalyst demonstrated 93.5% COD removal from the starting concentration of 2152 mg/L in 3 h. The photocatalyst showed strong recyclability and photostabiliy. Both dead-end and cross-flow PMRs can be used in photocatalytic systems. In the dead-end configuration, the whole stream is filtered by passing through a membrane (permeate). As a consequence, the concentration of the nontransferable components rises, resulting in creation of a filter cake on a membrane's surface, as well as a reduction in membrane permeability and photocatalytic efficiency. In the absence of turbulency, e.g., stirring, there will be insufficient contact between the contaminants, photocatalyst, and the light source [152]. Wang et al. [153] investigated a novel photocatalytic membrane created via pressure-driven filtration load with a ZnO/N-g-C 3 N 4 composite via glutaraldehyde as a crosslinker. SEM, XPS, and FTIR were employed to verify the photocatalyst loading on membrane surface. They found that in both immersion and filtration models, the photocatalytic capabilities of the ZnO/N-g-C 3 N 4 composite membrane were effective for the decomposition of tetracycline, ofloxacin, and ciprofloxacin under visible light (>420 nm). For tetracycline at 5 mg/L and 10 mg/L concentrations a ZnO/N-g-C 3 N 4 loading of 1.12 g/cm 2 resulted in 100% and 80% degradation, respectively. They concluded that a pro-longed reaction time on the membrane surface, low trans-membrane pressure (0.005 MPa), and narrow membrane size were advantageous for the elimination of antibiotics in the filtration processes.

Photocatalyst Type and Characteristics
Key parameters that significantly affect photocatalytic efficiency include the type of photocatalyst, the photocatalyst's physicochemical properties (band gap energy, particle size distribution, crystallographic structure, and chemical makeup), and the photocatalyst's concentration in the reacting environment. As mentioned previously, photons by energy equal to or greater than the band gap energy can be absorbed in photocatalytic activities, resulting in the creation of electron-hole pairs. However, photocatalysts that require visible light to function are more intriguing, and it has become a challenge for PMR systems to use a light source that is both environmentally friendly and economically viable [154]. For example, TiO 2 -supported photocatalyst is the most frequently employed in PMR in suspended form due to its great photochemical stability in aquatic solutions, robust catalytic activity, reasonably long lifespan of electron-hole pairs, low cost, and low toxicity. This material is inactive when exposed to visible light. As a result, TiO 2 can only absorb around 5% of the solar radiation that is in the UV spectrum [155]. Ahmad et al. [156]. stated that a developed composite ceramic membrane may benefit from a synergy of dead-end filtration and cross-flow filtration while being subjected to intermittent UV irradiation in order to efficiently prevent membrane fouling. To remove organic dye impurities in a photocatalytic membrane reactor, a partly coated TiO 2 (pc-TiO 2 ) layer was made with the assistance of cheap polyvinyl chloride (PVC) to make gaps in a porous Al 2 O 3 membrane substrate. Their study revealed that the pc-TiO 2 /Al 2 O 3 composite membrane has superior water flux and anti-fouling capabilities compared to the uniformly coated TiO 2 /Al 2 O 3 (UC-TiO 2 /Al 2 O 3 ) membrane. The photocatalytic activity of UC-and pc-TiO 2 /Al 2 O 3 composite membranes was significantly enhanced during cross-flow membrane filtration in comparison to that of the Al 2 O 3 bare membrane substrate.

Light Source
When light is shone on photocatalysts, photons with energy higher than or equal to the band gap are absorbed, valence band electrons are shifted to the conduction band. The oxidation and reduction reactions that occur are due to the production of electron-hole pairs [4]. Consequently, the kind and intensity of light have a significant impact on the performance of photocatalysis [157].
The sol-gel procedure and the dip-coating may be used to create a nanostructured TiO 2 film from titanium tetraisopropoxide. Sol-gel nanostructured TiO 2 (anatase phase) film was investigated for its photocatalytic degradation of azithromycin to determine the most efficient degradation pathway for application in wastewater treatment. At the pH of 10 and UV-C irradiation maximum degradation was achieved. The LED irradiation source with emission wavelength of 365 nm was not as efficient as the UV-C lamp. The LED bulb, however, may be a "real-world" option due to its low price, high energy efficiency, and low environmental impact [158]. Shang et al. [159]. investigated the antibacterial activities of TiO 2 photocatalysts under various light sources, exclusively under visible light. They discovered that by doping metal ions and nonmetal ions on TiO 2 and compounding with polymers, they could increase the photocatalytic activity to the visible light region, improve the surface characteristics, and enhance the contact area with bacteria. Reactive oxygen species (ROS) and hydroxyl free radicals damage the cell membrane, DNA, and enzymes.

Degradation of Pharmaceutical Compounds via PMR
Pharmaceutical compounds are structurally complex and environmentally stable. They typically contain abundant aromatic rings. Thus, conventional wastewater treatment methods cannot effectively remove them. Hence, the use of PMRs has become a popular solution for removal of pharmaceutical compounds from water. Fang et al. [153] synthe-sized a ZnO/N-g-C 3 N 4 composite and immobilized it on a commercial polymer membrane via GA as the crosslinker to breakdown antibiotics under visible light in a PMR setup. Additionally, the ZnO/N-g-C 3 N 4 composite photocatalytic membrane properties were evaluated using the immersion model and the filtration model. The amount of ZnO/Ng-C 3 N 4 loading and GA concentration were significant for the photocatalytic abilities of composite membranes, according to an immersion model. The outcome demonstrated that the filtering model results in a greater antibiotic decomposition at longer photocatalytic reaction time, that could be attained by narrow membrane pore sizes, low TMP, and reduced flow. Using a similar photocatalytic membrane fabrication procedure using pristine membrane with MWCO of 50 kDa, improved TC degradation (71.7%) was obtained by the immersion approach which is attributed to the decreased flux and increased retention time. The Photocatalytic membrane degradation of pharmaceutical compounds is shown in Table 7.

Degradation of Dye Compounds via PMR
Synthetic paint is one of the most abundant pollutants in sewage and effluent of industrial plants. The most commonly investigated dyes are rhodamine B, methylene blue, and methyl orange. Conventional wastewater treatment methods are inefficient in removing dyes. Photocatalytic technology is one of the most successful approaches suggested for dye removal.
Dzinun et al. [180] create a TiO 2 -PVDF photocatalytic membrane by addition of different loading of TiO 2 nanoparticles on a PVDF membrane for methylene blue removal. They characterized the resulting photocatalytic membranes with FE-SEM, EDS, and AFM. The results of their custom-designed PMR demonstrated that adding TiO 2 nanoparticles to the PVDF membrane speeds up methylene blue removal from waste water. Kolesnyk et al. [181] studied the impact of g-C 3 N 4 loading on a commercial PVDF membrane on rhodamine-B removal. The highest removal rate occurred in the alkaline medium. In this study, the membrane lost approximately 15% of its pores after five cycles (a total of 50 h). Yu et al. [163] synthesized a PSF membrane coated with g-C 3 N 4 and TiO 2 nanocomposites to investigate the removal of sulfamethoxazole. They reported that sulfamethoxazole was converted into seven other non-toxic substances using their custom-designed PMR with sun light irradiation [163]. Horowitz et al. [179] investigated the efficiency of Al 2 O 3 coated membranes with pore sizes of 200 and 800 nm for carbamazepine removal at different operating conditions. It was observed that the PMR efficiency under UV irradiation is much higher than that under visible waves. They also reported that the contaminant removal rate increases with temperature. Ma et al. [182] investigated humic acid removal from wastewater using TiO 2 /Al 2 O 3 photocatalyst and membrane microfiltration processes. They found that light intensity significantly affects humic acid removal. A summary of dye degradation via PMRs is reported in Table 8.

Degradation of Hydrocarbons via PMR
Rani et al. used a membrane photocatalytic reactor containing suspended TiO 2 photocatalytic particles for naphthalene removal. They studied the impact of different operating parameters such as the initial concentration of naphthalene (5-25 mg/L), photocatalyst loading (0.1-0.9 g/L), and pH (3-9) on naphthalene removal rate. The maximum naphthalene removal by separate photocatalysis and membrane was 76.8% and 49.1%, respectively, while a naphthalene removal of 90.2%. could be achieved by PMR [183]. Batch PMRs functioning in the dead-end mode was designed by Moslehyani et al. [184] which could to eliminate 99% of the hydrocarbons from sludge after 2 h. Ag-TiO 2 -coated alumina membrane in a dead-end configuration to degrade rhodamine rate of 1.007 mg/m 2 h 1 [185]. Despite the promising outcomes, the researchers emphasized that the dead-end process leads to the buildup of separated substrates on the membrane surface and ultimately forms a cake layer, which decreases photocatalytic efficiency. A summary of hydrocarbon degradation via PMRs is reported in Table 9.

Degradation of Other Pollutants via PMR
Pollutants such as toxins, detergents, and heavy metals that do not fall into the above categories are also treated by PMRs which can be seen in Table 10. For example, it can be seen from Table 10, 99.9% of toxic hexavalent chromium (Cr (VI)) at an initial concentration of 10 mg/L could be removed by PMR containing Chitosan-sodium alginate/Fe-doped WO3 photocatalyst and PES membrane after 240 min using a 300 W Xe light source [186].  [193] Methylene blue 500 Co/PC/g-C 3 N 4 Xe Lamp 300 W 360 99 [194] Methylene blue 1 NbCxOy/NbOx/ g-C 3 N 4 --480 100 [195] RhodamineB 10

Membrane Fouling in PMRs
Different components of feed, such as organic substances, photocatalysts, colloids, salts, and cells, can have varying effects on system performance [229]. The fouling of PMR membranes is caused by the deposition of feed components on the membrane. Photocatalytic oxidation partially eliminates foulants [230]. Foulant adhesion, pore blocking, the cake layer formation, and temporal and spatial changes of foulant structure during long-term filtration are among factors that promote fouling [230,231]. Photocatalyst nano particles may form microaggregate and deposit on the membrane during filtration. Organic pollutants accumulation in the vicinity of membrane may result in formation of a very thin layer leading to substantial pore clogging and flow rate reduction. Adsorption of organic contaminants on TiO 2 particles and its composites such as Degussa P25 TiO 2 , Ca alginate polymer/TiO 2 fibers, nano-structured TiO 2 /silica gel photocatalyst, and titanium tetraisopropoxide in the absence of effective UV absorption may result in formation of a dense cake layer on the membrane surface and further flow rate reduction [232].
Designing non-fouling membranes is highly sought. Zheng et al. [233] combined the cellulose nanocrystals (CNCs) onto Cu-MOF-74 by physically stirring, and then coated the composites on the membrane to enhance the antifouling efficiency of PVDF membranes. CNC/Cu-MOF-74 composite coating on PVDF membrane increased its hydrophilicity, which in turn considerably improved the membrane's permeability and productivity. They also reported enhanced electrostatic repulsion based on the contact angle test and Zeta-potential measurement. Due to Cu-predominate MOF-74's antibacterial activity, the CNC/Cu-MOF-74 modified membrane also demonstrated increased antibacterial performance. effective antibacterial performance of composite membrane was attributed to Cu 2+ release and •OH production.

Reactor Design
Modulating photocatalytic reactions, exchanging catalysts, and degrading pollutants are all easier with slurry PMRs. Although catalyst separation is not required for IPMRs, the catalyst loading cannot be tailored to the feed's specific composition; higher catalyst loading, larger membrane surface area, and higher-pressure drop are required which increases the reactor volume, energy consumption of pumps, and process cost. Exchanging the catalyst is also a challenging process especially in IPMRs [234,235].

Photocatalyst Loading
Membrane foulants can be reduced and photocatalytic degradation may be accelerated by increasing catalyst loading due to increase reaction surface area. [234]; however, photocatalyst loading has an optimum value for any specific process after which increased opacity of the reaction mixture hinders light absorption by photocatalyst [13,236]. Elevated photocatalyst loading results in reduced foulant degradation which enhances fouling rate on the membrane [237,238].

Conclusions and Future Perspectives
This review described various hybrid photocatalysis and membrane process designs for removing organic contaminants from water. The main advantage of PMRs is reten-tion and reuse of photocatalyst. The advantages and disadvantages IPMRs and SPMRs, as well as important design/operation parameters were discussed. The performances of a photocatalytic reaction can be improved by utilizing a suspended photocatalyst as opposed to an immobilized one, owing to larger active surface and subsequently improved photocatalyst-substrate interaction. SPMR empowered by air bubbles and effluent flushing appears to be more suitable for treating water and wastewater. Visible light may be utilized as an irradiation source and efficient solar-driven photocatalytic conversion has emerged in recent years. The appropriate selection of the membrane is crucial, as it must have great permeability to the desired product and retain contaminants and photocatalyst to facilitate the rapid removal of the product from the reaction environment. When it comes to treating wastewater, TiO 2 -based PMRs excel because of their excellent separation efficiency and low maintenance requirements. When designing visible-light-operated photocatalysts, it is important to examine the option of employing the sun as a clean, low-cost light source to make the process more environmentally friendly. Advantages of using PMRs for the partial oxidation and reduction of organic matter include (a) extending the lifetime of polymeric membranes with the help of visible light as a source of radiation, and (b) enhancing photocatalyst recovery through the use of novel materials in the synthesis of photocatalyst composites and semiconductor coatings on optical fibers. PMRs utilization has become a mode viable wastewater treatment option upon developments in photovoltaic technology (solar energy conversion) and the use of LED lamps (UV and/or visible). Data Availability Statement: All data generated or analyzed during this study are included in this published article.

Conflicts of Interest:
The authors declare no conflict of interest.